Platinum-Group-Metal-Free Electrocatalysts Via N⁺ Ions Implantation and Iron Evaporation Over 2D-Supported Carbonaceous Materials

Ion beam methods (IBM) for selective ion implantation are at the basis of large-scale production of fine-tuned optically related materials and semiconductors but also for p- and n-doped carbon nanotubes and graphene [1–4]. Continuous research on cost-effective nanomaterials for energy conversion in fuel cells, zinc-air batteries, and electrolyzers could actually benefit from a precise and scalable approach for the preparation of electrocatalysts. Indeed, platinum-group-metal-free electrocatalysts (PGM-free), specifically designed for those applications, are materials composed of carbon, nitrogen, oxygen, and non-noble transition metals, hence chemically compatible with IBM. Commonly, PGM-free are synthesized through a chemical process involving both organic and inorganic compounds [5–7]. To enhance catalytic efficiency, these compounds are mechanically mixed and subjected to one or more pyrolysis cycles at high temperatures (&gt;900°C) in a controlled, non-oxidative atmosphere. Additionally, acid-washing steps are employed to remove unwanted secondary species, particularly for applications in low-pH environments, while also creating a porous structure [8]. Although the chemical synthesis method is widely used for producing PGM-free materials, it offers limited control over the final product, especially in terms of the distribution of oxygen and nitrogen functional groups.<br/>In an effort to apply IBM methodology to PGM-free materials, we investigated the feasibility of replicating the formation of active sites using a cleaner, physical approach, rather than the traditional pyrolysis-based route. The newly developed method is based on nitrogen ion (N+) implantation using a Kaufman ion source at varying beam energies, followed by iron deposition via electron beam evaporation in an ultra-high vacuum (UHV) clean chamber. After the first investigation performed over vertically aligned carbon nanotubes (VACNT), to pave the way and investigate the feasibility of producing fine-tuned PGM-free electrocatalysts without relying on nitrogen-containing precursors, we implemented the study. With the original recipe developed for VACNT, other carbon nanostructures can be investigated, like graphene grown on copper using chemical vapor deposition (CVD) technology. After preparing a set of samples at increasing ion beam energies (20–50 eV), along with control samples, preliminary analyses using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Raman spectroscopy revealed that the final product exhibited similarities to those produced by chemical methods. This indicates that the formation of active sites is a natural result of nitrogen being incorporated into the carbon matrix. Moreover, by simply adjusting the ion beam energy, it was possible to maximize the content of pyridinic nitrogen while minimizing other nitrogen moieties. Finally, electrochemical testing of a representative sample demonstrated catalytic activity toward the oxygen reduction reaction (ORR), confirming the viability of the process. These initial results pave the way for the development of PGM-free electrocatalysts through a nitrogen-compound-free synthesis, offering a sustainable approach to creating critical raw-material-free nano-electrocatalysts.<br/><br/><b>References</b><br/>[1] Appl. Phys. Lett. 97 (2010) 183103. https://doi.org/10.1063/1.3507287.<br/>[2] Nano Lett. 10 (2010) 4975–4980. https://doi.org/10.1021/nl103079j.<br/>[3] New Carbon Mater. 28 (2013) 81–86. https://doi.org/10.1016/S1872-5805(13)60068-2.<br/>[4] Nanomaterials 9 (2019) 425. https://doi.org/10.3390/nano9030425.<br/>[5] J. Electrochem. Soc. 166 (2019) F3136. https://doi.org/10.1149/2.0201907jes.<br/>[6] J. Solid State Electrochem. 25 (2021) 93–104. https://doi.org/10.1007/s10008-020-04537-x.<br/>[7] Nanoscale 13 (2021) 4576–4584. https://doi.org/10.1039/D0NR07875A.<br/>[8] Joule 4 (2020) 33–44. https://doi.org/10.1016/j.joule.2019.12.002.